1. Field of the Invention
The present invention relates to a hydraulic fluid supply structure configured to supply pressurized fluid from a hydraulic pump to a hydraulic clutch mechanism for engaging and disengaging power transmission to a working machine, and to a raising and lowering hydraulic actuator for raising and lowering the working machine.
2. Related Art
It is conventionally known to supply pressurized fluid from a single hydraulic pump to a hydraulic operating type clutch mechanism for engaging and disengaging power transmission to a working machine, and to a raising and lowering hydraulic actuator for raising and lowering the working machine (see e.g., Japanese Laid-Open Patent Publication No. 2005-1636).
Such conventional hydraulic fluid supply structure allows the hydraulic fluid sources of the hydraulic clutch mechanism and the hydraulic actuator to be common; thereby it is effective in reducing cost and realizing miniaturization.
However, the conventional hydraulic fluid supply structure has a drawback in that the raising speed of the working machine by the hydraulic actuator cannot be increased.
Specifically, the hydraulic fluid supply structure has an external hydraulic fluid extraction line for supplying the hydraulic fluid to the hydraulic actuator and a PTO hydraulic fluid line for supplying the hydraulic fluid to the hydraulic operating type clutch mechanism, the external hydraulic fluid extraction line being fluidly connected to a discharge line of the hydraulic pump through a resistance valve, the PTO hydraulic fluid line being fluidly connected to the discharge line through a throttle valve or an orifice.
The hydraulic fluid supply structure has a PTO switching valve interposed in the PTO hydraulic fluid line, wherein the supply of the hydraulic fluid to the hydraulic clutch mechanism is on-off controlled by the PTO switching valve.
Specifically, the conventional configuration is configured so that the PTO hydraulic fluid line fluidly connects to a drain line when the PTO switching valve turns OFF the supply of hydraulic fluid to the hydraulic clutch mechanism.
That is, in the conventional configuration, the PTO hydraulic fluid line is opened to the drain line, when the hydraulic fluid supply to the hydraulic clutch mechanism is stopped so that the hydraulic clutch mechanism is in a disengaging state.
In the conventional configuration, the power transmission to the working machine is normally disengaged (i.e., supply of hydraulic fluid to the hydraulic clutch mechanism is stopped) in view of safety and the like, when raising the working machine by the hydraulic actuator. The PTO hydraulic line is opened to the outside in the conventional configuration as described above, when supply of hydraulic fluid to the hydraulic clutch mechanism is stopped. Therefore, some of the pressurized fluid is constantly leaked out from the discharge line through the drain line.
Therefore, it takes time to increase the amount of fluid and raise the hydraulic pressure in the external hydraulic fluid extraction line, whereby a great amount of time is required in the raising operation of the working machine.